Turbine stages, especially end stages of conventional turbo machine have long rotor blades. The last stage rotor blades have interlocking shrouds to improve in particular vibrational behavior. Essentially, a shroud has thickness and has sides, which are cut to create an interlocking configuration when adjacent rotor blades are present.
The purpose of a shroud is to prevent leakage over the blade tip, improve efficiency of the turbine and improve the dynamic and vibration qualities of the rotor blade. The interlocking of shrouds takes place along two bearing faces. The interlocking of shrouds at bearing faces leads to dampening of vibrations. An additional feature is provided on the tip of a rotor blade shroud is a fin. Depending upon the size of the blade shroud, one or more fin may be present.
The fins have a sealing function to reduce secondary flow across the blade tips. Bending stiffness required to withstand centrifugal loads, which are generated during the movements of blades, is provided by the fin height.
Presently, shrouds for last stage rotating blades are essentially solid. The shroud is an additional load to the blade and the rotor. The airfoil and root of the blade carry the weight of the shroud. It has significant impact on cross sectional area of the airfoil and consequently on the weight of airfoil and root. During operation, as blades rotate at high speeds on a rotor about the turbine axis the blades are held in the rotor by the blade root, which mechanically engages in the rotor. As the blades rotate, the centrifugal forces cause the blade to pull in radial direction and to load the rotor.
The amount of loading on the rotor and hence the root, which holds the blade in the rotor is a function of the blade weight. A heavy blade leads to more stresses on the interface between blade root and rotor, and to high total radial forces on the rotor. The weight of shrouds increases the radial force, which approaches the rotor limit. Therefore, it poses important design limitations to the performance of a turbine and can reduce the overall life of the root and rotor.
Turbo machines, especially steam turbines, have long blades to increase the exhaust annulus area for performance reasons. The annulus area is increased to allow high mass flows. Long blades are used for large annulus areas, which result in higher weight for blades. Current designs typically have fully shrouded tips of blades with fins for improved vibration control and to reduce the tip leakage losses.
To reduce leakages during turbine operation a honeycomb is typically arranged opposite to the fin. During operation the fin cuts into the honeycomb.
The efficiency of modern turbines and compressors depends upon a tight seal between the rotating components (blades) and the stationary component. This seal is established by allowing the fins of blades to cut (abrade) a groove in an abradable seal material, which prevents a substantial volume of air from leaking past the blade tip. Typically the seal materials are honeycombs seals or have sintered metallic particles and brazed in place. To assure a safe operation of the turbine, the fin has to be sufficiently strong to cut into the seal material under operating conditions.
Further, the fin has to be sufficiently strong to fulfill its dampening function when the fins of adjacent blades bear on each other during operation.
To avoid creep of the fins during hot operating conditions, and to increase the lifetime cooling of fins has been suggested in DE19904229. It was further noted, that the weight of the fins could be reduced by drilling holes into the fin. However, the achievable weight reduction by drilled holes is limited. Further, holes can be detrimental to the lifetime of the blade, as they have a notching effect, which can lead to stress concentration and consequently to high local maxima in the stress distribution in the fin.